Understanding Web Communications

Like all client-server applications, web applications have two distinct components:

Client

Client also known as the front-end interface, the web browser presents the user interface, accepts user input, and sends data to the server for processing.

Server

Server also known as the back end, the web server responds to requests from clients for specific pages. It responds with an HTML page that includes instructions for how to generate the user interface.

The web browser (the client) and the web server communicate by using Hypertext Transfer Protocol (HTTP), a text-based network protocol assigned to TCP port 80. If the server has a certificate, the client and server can use HTTP Secure (HTTPS) to authenticate the server and encrypt communications. HTTPS is assigned to TCP port 443.

 Web Communications typically follow these steps:

• A user enters an address, such as http://www.microsoft.com/, into the web browser.
• The web browser connects by using HTTP and sends a GET request, such as GET / (to retrieve the root page), to the web server.
• The web server processes the requested page. This action might cause the server to return static HTML or image files, or it might execute ASP.NET code that performs workflow tasks or connects to a database.
• The web server uses HTTP to send a response back to the web browser. If the request was processed successfully, the web server returns the HTTP status code 200, along with an HTML document. If the server cannot find the page, it returns the code 404. If the user requests an outdated or relocated page, the server returns the code 302 and the new URL so that the browser can access the correct page. This is known as redirection. Several other responses are possible as well, depending on the particular situation.
• The user’s web browser then processes the response by displaying the HTML page (if the code was 200), showing an error message (if the code was 404), or loading a different page (if the code was 302). Other server responses are similarly handled by the browser, depending upon the response.

This process is repeated each time the user clicks a button or link.

For more detail you can refer:  http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-515